Thus far, a CMOS solid state imaging element and a CCD have been known as two-dimensional solid state imaging elements, and single-crystal silicon (Si) is generally used for the light absorbing layer of the light detecting element of them that performs photoelectric conversion.
Si is an indirect transition semiconductor and has a band gap of 1.1 eV, and therefore has sensitivity to visible light wavelengths to near-infrared wavelengths of approximately 1.1 um (micrometers). However, due to the wavelength dependence of the light absorption coefficient, the light absorption efficiency per unit thickness becomes smaller as the wavelength becomes longer.
For example, in the case of a solid state imaging element in which the thickness of the Si layer as the light absorbing layer is 3 um, the light absorption efficiency at a wavelength of 650 nm is approximately 60 to 70%, whereas, at a wavelength of 900 nm, the light absorption efficiency is only approximately 10 to 20% and most photons are transmitted through the Si layer. Hence, when it is attempted to obtain a solid state imaging element having a high sensitivity to light in the red to infrared region, increasing the thickness of the Si layer is known as an effective method.
However, increasing the thickness of the Si layer has a high degree of manufacturing difficulty, such as the need to perform high energy implantation in order to obtain a desired impurity profile, and furthermore directly leads to an increase in material cost. In addition, the ratio of the thickness to the pixel size of the solid state imaging element is increased, and an increase in the amount of color mixing components of the Si bulk in the Si layer etc. are caused; thus, this is a factor in the degradation of image quality. Moreover, an increase in the amount of defects in the crystal etc. due to the increase in the thickness of the Si layer are factors in the degradation of pixel characteristics, such as an increase in dark current and white spots.
In this regard, as a method for obtaining a high sensitivity to light on the long wavelength side without increasing the thickness of the Si layer, a structure in which the loss of light caused by an etalon phenomenon based on the interference of light is suppressed by forming a fine, random concave-convex structure on the surface on the opposite side to the light receiving surface of the pixel of the solid state imaging element is proposed (e.g. see Patent Literature 1).